The present invention relates to multi-stage Gifford McMahon (GM) type pulse tube refrigerators as applied to recondensing helium in a MRI magnet. When a conventional multi-stage pulse tube is operated in the neck tube of a MRI cryostat, where it is surrounded by helium, significant thermal losses can occur due to convective circulation of the helium because of differences of the temperature profiles in the pulse tubes and the regenerators.
GM type refrigerators use compressors that supply gas at a nearly constant high pressure and receive gas at a nearly constant low pressure to an expander. The expander runs at a low speed relative to the compressor by virtue of a valve mechanism that alternately lets gas in and out of the expander. Gifford, U.S. Pat. No. 3,119,237, describes a version of a GM expander with a pneumatic drive. The GM cycle has proven to be the best means of producing a small amount of cooling below about 20 K because the expander can run at 1 to 2 Hz.
A Pulse Tube refrigerator was first described by Gifford in U.S. Pat. No. 3,237,421, which shows a pair of valves, as in the earlier GM refrigerators, connected to the warm end of a regenerator, which in turn is connected at the cold end to a pulse tube. Early work with pulse tube refrigerators in the mid 1960s is described in a paper by R. C. Longsworth ‘Early pulse tube refrigerator developments, Cryocoolers 9, 1997, p. 261-268. Single-stage, two-stage, four stage with inter-phasing, and co-axial designs were studied. All had the warm ends of the pulse tube closed and all but the co-axial design had the pulse tubes separate from the regenerators. While cryogenic temperatures were achieved with these early pulse tubes the efficiency was not good enough to compete with GM type refrigerators.
A significant improvement in pulse tube performance was reported by Mikulin et al, ‘Low temperature expansion (orifice type) pulse tube, Advances in Cryogenic Engineering, Vol. 29, 1984, p. 629-637, and much interest ensued in looking for further improvements. This initial improvement used an orifice and a buffer volume connected to the warm end of the pulse tube to control the motion of the “gas piston” in the pulse tube to produce more cooling each cycle.
Subsequent work focused on both means to improve the control of the gas piston and on improving the configuration of the pulse tube expander. S. Zhu and P. Wu, in a paper titled ‘Double inlet pulse tube refrigerators: an important improvement’, Cryogenics, vol. 30, 1990, p. 514, describe a double orifice means of controlling the gas piston. Gao, U.S. Pat. No. 6,256,998 describes a means of controlling the gas pistons in a two-stage pulse tube that works well at 4 K.
Multi-stage pulse tubes were first investigated by Gifford and Lonsworth ‘Early pulse tube refrigerator developments’, Cryocoolers 9, 1997, p. 261-268 using a design that pumped heat from one stage to the next higher stage. Chan et al. found that it is possible, and better, to have the second stage pulse tube extend all the way from the cold heat exchanger to ambient temperature as described in U.S. Pat. No. 5,107,683.
This concept is one of several configurations reported by Y. Matsubara, J. L. Gao, K. Tanida, Y. Hiresaki and M. Kaneko, ‘An experimental and analytical investigation of 4K (four valve) pulse tube refrigerator’, Proc. 7′ Intl Cryocooler Conf., Air Force Report PL-(P-93-101), 1993, p. 166-186, and by J. L. Gao and Y. Matsubara, ‘Experimental investigation of 4 K pulse tube refrigerator’, Cryogenics 1994, Vol. 34, p. 25. It has proven to work well for two-stage 4 K pulse tubes. The arrangements that were studied all had the pulse tubes separate from the regenerators and parallel to them, with the cold end oriented down. This is the most common configuration of present day two-stage pulse tubes and is referred to herein as the conventional design. U.S. Pat. No. 5,412,952, Ohtani et al., shows a two-stage pulse tube with a thermal link between the first stage heat station and the adjacent second stage pulse tube. One of the present inventors tested this configuration in 1994 and found no improvement in cooling performance, but it did cause a change in the pulse tube temperature profile.
Temperature differences between the pulse tubes and the regenerators are not a problem when the tubes are separate from the regenerator and the pulse tube is surrounded by vacuum. The temperature differences however result in convective thermal losses when a conventional pulse tube is mounted in the helium atmosphere in the neck tube of a MRI cryostat.
Losses associated with temperature differences between the pulse tube and regenerator were addressed in connection with co-axial pulse tubes by Inoue in JP H07-260269. This patent shows several porous plug heat exchangers spaced inside the pulse tubes near the warm end and in contact with the walls of the first stage regenerator. U.S. Pat. No. 5,613,365, Mastrup et al., describes a single stage concentric (co-axial) pulse tube in which a central pulse tube has a thick wall made of low thermal conductivity material that provides a high degree of insulation from the annular regenerator on the outside. Rattay et al. extended this idea in U.S. Pat. No. 5,680,768, in which the surrounding vacuum extends into a gap between the pulse tube wall and the inner wall of the regenerator.
Another means of insulating the wall of a pulse tube is described by Mitchell, U.S. Pat. No. 6,619,046. Studies of losses in co-axial pulse tubes are reported in papers by L. W. Yang, J. T. Liang, Y. Zhou, and J. J. Wang. ‘Research of two-stage co-axial pulse tube coolers driven by a valveless compressor, Cryocoolers 10, 1999, p. 233-238, and by K. Yuan, J. T. Liang, and Y. L. Ju, ‘Experimental investigation of a G-M type co-axial pulse tube cryocooler”, Cryocoolers 12, 2001, p. 317-323. Losses were minimized by superimposing “dc” flow that brought warm gas down the pulse tubes over many cycles.
Zhou et al., U.S. Pat. No. 5,295,355, describe a multi-bypass pulse tube that has as its objective an improvement in efficiency. In effect it is a multi-stage pulse tube but there is only one pulse tube. In practice it is nearly impossible to implement because of the difficulty of having the exact same amount of gas flow in both directions through each by-pass orifice. It does have the characteristic of imposing essentially the same temperature profile in the pulse tube as the regenerator.
Problems associated with recondensing helium in a MRI magnet were addressed by Longsworth in U.S. Pat. No. 4,606,201. A two-stage GM expander that has a minimum temperature of 10 K precools gas in a JT heat exchanger that produces cooling at 4 K. The JT heat exchanger is coiled around the GM expander so that the temperature of both the JT heat exchanger and the expander get progressively colder between the warm and cold ends. The expander assembly is mounted in the neck tube of a MRI magnet where it is surrounded by helium gas that is thermally stratified by virtue of being vertically oriented with the cold end down. The 4 K heat station has an extended surface to recondense He. Refrigeration is transferred to cold shields in the MRI cryostat at two heat stations which are at temperatures of approximately 60 K and 15 K. Mating conical heat stations and bellows in the neck tube enable both heat stations to engage as the warm flange is bolted down and sealed with a face type “O” ring.
Longsworth, U.S. Pat. No. 4,484,458, had previously described the concentric GM/JT expander which had straight heat stations and a radial type “O” ring seal at the warm flange. This permits the expander to be moved axially to establish a desired position of the expander heat stations relative to the neck tube heat stations.
Advances in pulse tube technology and MRI cryostat design now make it possible to use a two stage pulse tube to cool a single shield at about 40 K and recondense helium at about 4 K. Two-stage pulse tube expanders are preferred over two-stage GM expanders because they have less vibration and thus generate less noise in the MRI signal. When a pulse tube of the current design, with the pulse tubes parallel to the regenerators, is inserted into the neck tube of a MRI magnet it is found that helium gas in the neck tube circulates between the pulse tubes and the regenerators due to the temperature differences between them. This results in a serious loss of refrigeration.
Stautner et al., PCT WO 03/036207 A2, explains the problem for a conventional two stage 4 K pulse tube and offers a solution in the form of a sleeve that surrounds the pulse tube assembly and has insulation packed around the tubes. The sleeve has a heat station at about 40 K and a recondenser at the cold end. It can be easily removed from the neck tube to be serviced.
Another solution to the problem of convection losses of a conventional two-stage 4 K pulse tube in a MRI neck tube is offered by Daniels et al. in PCT WO 03/036190 A1. Insulated sleeves around the pulse tubes and regenerators reduce convective losses when the pulse tube is mounted in the helium gas in a MRI neck tube.
A conventional two-stage pulse tube refrigerator has the pulse tubes and regenerators in separate parallel tubes. In conventional pulse tubes that operate in vacuum the length and diameter of the pulse tubes and regenerators can be optimized almost independently of each other. When mounted in the neck tube of a MRI cryostat the helium in the neck tube results in thermal losses due to convection because of the temperature differences between the pulse tubes and the regenerators, thus other factors have to be considered in the design.
It is an object of this invention to minimize heat loss by convection of a pulse tube when it operates in a helium environment.